The present invention relates to a servo brake for motor vehicles comprising a pedal-operated tandem master cylinder including two master pistons, wheel brake cylinders connected thereto by way of at least one brake circuit, a brake valve connected between the pedal and the first master piston, which brake valve upon actuation by the brake pedal applies, in controlled manner, pressure medium supplied by a hydraulic pump connected to an intake reservoir to the first of the master pistons. Optionally, the pressure medium is supplied to a brake circuit directly to provide a controlled pressure. The servo brake further comprises a wheel slip brake control system which at an initial slip occurring on one or several motor vehicle wheels, will automatically reduce the brake force on the motor vehicle wheels to a value just permitting the wheels to turn. The brake control system includes a switch valve to which the controlled pressure is equally applied which switch valve is normally closed. The control system opens at an initial slip of one or more motor vehicle wheels to apply the controlled pressure of the master cylinder from where the pressure medium will be passed by way of two check valves to the pressure chambers located behind the master pistons to which pressure chambers the brake circuits are connected.
In conventional motor vehicles servo brakes of this type, a common switch valve is provided for both master piston pressure chambers which will open upon the initial wheel slip thereby supplying the controlled pressure, by way of the check valves, directly into the two brake circuits to replace there used-up pressure medium directly by the pump and the accumulator respectively.
The disadvantage envolved with the conventional system is that in the event of a failure of a brake circuit, (for example, due to leakage) upon occurrence of a wheel slip, pressure medium is continuously supplied into the defective brake circuit which could exceed the capacity of the pump provided thereby constituting a safety risk. If in case of such a failure the switch valve were turned off (that is, closed) no wheel slip brake control of the still intact brake circuit would be possible.
It is, therefore, the object of the present invention to provide a motor vehicle brake of the type described in which in the event of a failure of one of the two brake circuits to which pressure is applied by the master pistons, the supply of pressure medium into the defective brake circuit upon occurrence of a wheel slip will be avoided while the wheel slip control on the still intact brake circuit will be unaffected.